The Pacific Coast of Washington is at risk from tsunamis. These destructive waves can be caused by coastal or submarine landslides or volcanism, but they are most commonly caused by large submarine earthquakes.
Tsunamis are generated when these geologic events cause large, rapid movements in the sea floor that displace the water column above. That swift change creates a series of high-energy waves that radiate outward like pond ripples. Offshore tsunamis would strike the adjacent shorelines within minutes and also cross the ocean at speeds as great as 600 miles per hour to strike distant shores. In 1946, a tsunami was initiated by an earthquake in the Aleutian Islands of Alaska; in less than 5 hours, it reached Hawaii with waves as high as 55 feet and killed 173 people.
Tsunami waves can continue for hours. The first wave can be followed by others a few minutes or a few hours later, and the later waves are commonly larger. The first wave to strike Crescent City, California, caused by the Alaska earthquake in Prince William Sound in 1964, was 9 feet above the tide level; the second, 29 minutes later, was 6 feet above tide, the third was about 11 feet above the tide level, and the fourth, most damaging wave was more than 16 feet above the tide level. The third and fourth waves killed 11 people. Estimates of the damage range from $7.4 million to $16 million (in 1964 dollars). That same tsunami destroyed property in many areas along the coast from Alaska to California. In Washington alone, that tsunami caused $105,000 (in 1964 dollars) in damage.
That 1964 event was the most recent significant tsunami to reach the Washington coast, but recent geologic investigations indicate that large tsunamis have struck our coast many times in the last few thousand years.
On the Pacific Coast, from southern British Columbia to northern California, people and property are at risk both from distantly and locally generated tsunamis. Recent studies indicate that about a dozen very large earthquakes (with magnitudes of 8 or more) have occurred in the Cascadia subduction zone west of Washington. Computer models indicate that tsunamis waves generated by these local events might range from 5 to 55 feet in height and could affect the entire coastal region.
When an earthquake that might generate a Pacific Coast tsunami is detected, the Alaska Tsunami Warning Center calculates the danger to the northeast Pacific Coast and notifies the communities at risk. Those warnings may give people a few hours to prepare and evacuate (depending on the distance to the earthquake).
If the earthquake occurs off our coast, however, there may be no time to send out hazard warnings. The first waves could arrive within minutes of the earthquake. The only tsunami warning might be the earthquake itself.
In order to plan for hazards, citizens need to know what to expect. In the last few years, there have been significant advances in understanding the earthquakes that have occurred on the Cascadia subduction zone and the tsunamis that have struck the Pacific Coast. This information is the foundation for planning efforts.
In order to form successful mitigation strategies, local planners and emergency managers need to know what has worked and what has not. In the last few years, many organizations and communities have addressed these issues. Those experiences can help others to formulate the best possible plans for the coastal communities.
[modified from Open File Report 98-4, "Tsunamis on the Pacific Coast of Washington State and Adjacent Areas--A Selected, Annotated Bibliography and Directory"]